Demix ParentNode.childElementCount

[Elchi3]

The IDL for childElementCount is this:

interface mixin ParentNode {
  readonly attribute unsigned long childElementCount;
};

Document includes ParentNode;
DocumentFragment includes ParentNode;
Element includes ParentNode;

Therefore we need these pages (created in this PR):
http://localhost:3000/en-US/docs/Web/API/Document/childElementCount
http://localhost:3000/en-US/docs/Web/API/DocumentFragment/childElementCount
http://localhost:3000/en-US/docs/Web/API/Element/childElementCount

Currently existing is: https://developer.mozilla.org/en-US/docs/Web/API/ParentNode/childElementCount
(I made that a redirect to http://localhost:3000/en-US/docs/Web/API/Element/childElementCount which is the oldest/original API.)

This note was fun (I removed it). ElementTraversal seems to be the old mixin here. Neither ElementTraversal nor ParentNode are really interesting to readers as the note points out.

This property was initially defined in the ElementTraversal pure interface. As this interface contained two distinct set of properties, one aimed at Node that have children, one at those that are children, they have been moved into two separate pure interfaces, ParentNode and ChildNode. In this case, childElementCount moved to ParentNode. This is a fairly technical change that shouldn't affect compatibility.